444 



MUSCLE PHENOMENA. 



stimulation; the result in either case being a shortening or con- 

 traction of the muscle, which may be made manifest by some 

 device attached to the tendon of the muscle. 



Battery. For the generation of the current the Daniell cell 

 (Fig. 246) is the one best adapted and most commonly employed. 

 In it polarization is prevented and its constancy is very great. 

 Polarization consists in a diminution in the intensity of the cur- 

 rent, caused by a film of hydrogen which forms on the copper plate. 

 The Daniell cell consists of a glass jar holding dilute sulphuric acid 

 or a solution of copper sulphate, in which is a sheet of copper of a 

 cylindric form. Within the latter is a porous jar containing a solu- 

 tion of zinc sulphate, within which is a zinc prism. To keep the 

 solution of copper sulphate saturated, crystals of this salt are placed 

 in a perforated pocket attached to the copper plate. The action of 

 the sulphuric acid upon the zinc results in chemical changes by which 

 a current of electricity is generated when the zinc and copper are 

 metallically connected. The current within the cell flows from the 

 zinc to the copper, while outside it flows from the copper to the zinc. 

 The zinc is the positive plate, and the copper the negative : but the 

 end of the wire which is connected with the copper is the positive 

 pole or anode, and that connected with the zinc plate is the 

 negative pole or kathode. When the unattached ends of these wires 

 connecting the zinc and the copper are brought into contact with 

 a nerve a current of electricity flows through the nerve, the direc- 

 tion being from the anode to the kathode. The wires are also 

 termed electrodes, though this term is more commonly applied to 

 the terminations of the wires attached to suitable holders. When 



the electrodes are brought 

 into communication through 

 the intervening nerve the 

 circuit is closed, and a con- 

 traction of the muscle oc- 

 curs ; when one of them, or 

 both, is removed from the 

 nerve the circuit is broken, 

 and another contraction fol- 

 lows ; or the same results 

 will follow if the muscle is 

 directly stimulated without 

 the intervention of the nerve. 

 Keys. A more conven- 

 ient method of stimulating 

 a nerve or muscle is by 

 placing the one or the other 

 upon the electrodes, which 



FIG. 247. Electric key. 



are not connected directly, but through the intermediary of a key. 

 When the key is open the circuit is broken, and when it is closed 



